In a typical electrostatographic reproduction process machine, a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is imagewise exposed in order to selectively dissipate charges thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to the latent image forming a toner powder image on the photoconductive member. The toner powder image is then transferred from the photoconductive member to a copy sheet. The toner particles are heated at a thermal fusing apparatus at a desired operating temperature so as to fuse and permanently affix the powder image to the copy sheet.
In order to fuse and fix the powder toner particles onto a copy sheet or support member permanently as above, it is necessary for the thermal fusing apparatus to elevate the temperature of the toner images to a point at which constituents of the toner particles coalesce and become tacky. This action causes the toner to flow to some extent onto the fibers or pores of the copy sheet or support member or otherwise upon the surface thereof. Thereafter, as the toner cools, solidification occurs causing the toner to be bonded firmly to the copy sheet or support member.
U.S. Pat. No. 7,228,082 discloses a belt fuser having a multi-Tap heating element, the disclosure of which is incorporated herein by reference in its entirety.
FIG. 1 is an enlarged schematic cross-sectional view of a typical belt fuser heater element comprised of a thermally conductive ceramic substrate layer 8, a low friction coating layer 7, having a conductor/heater interfaced thereon; and conductive resistive traces 4, 5 and 6; and a ceramic glazing electrical insulation layer 10. Power delivered to the heating elements 4, 5 and 6 causes them to heat up and the heat is then transferred through the thermally conductive ceramic substrate 8 and the low friction coating layer 7 to the belt. The heating elements are electrically isolated by the ceramic glazing 10.
FIG. 2 is a schematic diagram of a segmented ceramic heater wherein Segment 1, Segment 2 and Segment 3 correspond respectively to heating elements 4, 5 and 6 of FIG. 1. It can be seen that the heater is heated by applying voltage to one of three taps V1, V2, V3 along the resistive trace comprised of R1, R2 and R3. The voltage tap is selected when a thermistor detects a segment is under temperature. The control algorithm ensures that switching is done by a hierarchy starting at the last segment (furthest from the return tap, V3). If the resistances/unit length are even, the controls are generally acceptable. If the resistances are not even, such as the last segment is under powered, that segment cannot keep up because it cannot be independently controlled. In other words, only Segment 1 can be independently controlled when a voltage is applied to voltage tap V1, while when voltage is applied at V2, power is applied to both segment 1 and segment 2, and when voltage is applied at V3, all segments receive energy. A key metric is power per unit length (W/mm). To use the segmented heater of FIG. 2, under series hierarchy control, the heater must be designed such that each subsequent segment is of a higher resistance than the previous. This ensures the series controlled segment is not under powered.
Prior art belt fusers are designed such that R1, R2, R3 and V1, V2 and V3 have selected values wherein W/mm1=W/mm2=W/mm3. To maintain temperature uniformity, all segments are controlled to the same set point temperature. The power is distributed by powering V3 to return (RTN) when segment is low, else V2 to RTN when Segment 2 is low, else powering V1 RTN when segment 1 is low.
A particular problems results if manufacturing tolerances of the belt fuser heating elements allow R3 to be low and subsequently W/mm3 to be lower than W/mm2, and thus the temperature of Segment 3 would be too low and would not recover because it cannot be powered independent of Segment 1 and Segment 2.
In other words, as noted above, the Segments are respectively sized to match the sheets being run in the printing machine. (That is, Segment A is sized to match A5, Segments 1+2 match 8.5×11 letter short edge and Segments 1+2+3 match A4 long edge.) Segment A is switched on nearly continuously and Segments B and C would be switched on according to larger paper sizes being run. Typically, Segment B is run in combination with Segment A when A4 short edge paper is being run and Segments A, B and C are switched on when A3 or A4 long edge sheets are being run. Thus, if running A4 short edge sheets, A+B would be switched on and Segment C would be relatively cool. If A3 sheets are to be run directly after, Segment C has to be heated. But to heat Segment C, then Segments A+B+C must be series connected and by the time Segment C is running a temperature, Segments A and B have already increased well above what is needed.
Thus, there is a need for a multi-tap series resistance ceramic heater functioning as a belt fuser that can ensure that all composite segments can be maintained at a desired operating temperature.